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Meteorite Myths

These Meteorite Myths come from an article published by Space.com called The Top 5 Cosmic Myths. Some very interesting reading is presented below.

Myth 1: Meteors are heated by friction as they pass through the atmosphere.

This one makes sense, which is why it's so pernicious. But it's still wrong.

Meteoroids are tiny bits of dust, rock, ice or metal that have the unfortunate luck of having their orbits intersect the Earth's. When they pass through our atmosphere, they are heated so ferociously that they glow (and at this point are called meteors), and are visible for hundreds of miles.

However, it is not friction that heats them. Think of it this way: a space shuttle's tiles are extremely delicate; they crumble easily in your hand. If they were heated by friction as the shuttle de-orbits and enters the atmosphere at Mach 25, the tiles would disintegrate. That's not a very good design characteristic.

In reality, it isn't friction, but ram pressure that heats the meteoroid. When a gas is compressed it gets hot, like when a bicycle pump is vigorously used to inflate a tire. A meteoroid, moving at 33,500 mph (15 kilometers a second) or more compresses the air in front of it violently. The air itself gets very hot, which is what heats the meteoroid. That's the fact, not friction.

Myth 2: Meteors are still very hot when they hit the ground.

You'd expect that something heated up so much that it glows would still be hot a couple of minutes later. Actually, the situation is a bit more complicated.

The super-hot air in front of the meteoroid is not actually in contact with the particle. (A particle can still be referred to as a meteoroid as it races through the atmosphere, while "meteor" is meant to describe the whole glowing phenomenon.)

The meteoroid's quick motion sets up a shock wave in the air, like from a supersonic airplane. The shocked air sits in front of the meteoroid, a few centimeters away (depending on the meteoroid's size) in what's called a standoff shock. Between the shocked air and the surface of the meteoroid is a relatively slow-moving pocket of air.

The surface of the meteoroid melts from the heat of the compressed gas in front of it, and the air flowing over it blows off the melted portion in a process called ablation. The meteoroid's high velocity provides the energy for all this heat and light, which rob it of speed. When it falls below the speed of sound, the shock wave vanishes, the heating and ablation stop, and the meteoroid then falls rather slowly, perhaps at a couple of hundred mph (or a few hundred kilometers per hour).

It's still pretty high up in the atmosphere at this point, and takes several minutes to fall to the ground. Remember, this tiny bit of rock spent a long time in space, and the core is pretty cold. Also, the hottest parts were melted and blown off. Even more, the air up there is cold, which chills the rock as well.

All of these things together mean that not only is the rock not hot when it hits the ground, it can actually be very cold. Some meteorites (what a meteoroid is called after it impacts) have actually been found covered in frost!


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